PMF code design

In addition to the

• Root models must allow for multi-point root systems. This is provided when IUptake is implemented and the standard Root class is used. If an alternate Root model is used, it needs to allow for multi-point root systems.
• Plant models must provide CO2 impacts. If the user changes CO2 in the weather component the plant model should respond.
• Plant models must use the transpiration value provided by MicroClimate. Without this, intercropping will not be possible.